The present invention relates to a sheet feed apparatus for an electrostatic copying machine or the like.
Electrostatic copying machines typically comprise a cassette for holding a stack of copy sheets. Such cassette includes a bottom plate for supporting the stack which is raised so that the top sheet of the stack engages with feed rollers provided above the cassette. A corner separator is provided at the leading edge corners of the stack such that when the feed rollers are rotated the top sheet is fed toward the corner separators. The corner separators cause the leading edge of the copy sheet to flex and pop out of the cassette for feeding to register rollers and a transfer station.
A problem has existed heretofore regarding skew feed of the copy sheet. This occurs when the copy sheets are closer to one side of the cassette than the other or when the copy sheets are not stacked evenly in the cassette. This latter condition also results in a particular copy sheet being closer to one side of the cassette than the other. Under such conditions, the result is that one of the corner separators exerts a greater force on the leading edge of the sheet than the other corner separator so that a rotational moment is imparted to the copy sheet about its center of gravity. The rotation of the copy sheet results in skew feed such that the side edges of the copy sheet are not parallel to the feed direction. Assuming that the skewed copy sheet does not jam in the feed path of the copying machine, which often happens, the copy image will appear tilted on the copy sheet.
In the prior art system described above, a feed roller must be provided adjacent to each corner separator, and the rollers must be spaced equally from the side edges of the copy sheets to prevent rotational moments and skew feed. However, it is desirable to be able to produce copies on different sizes of copy sheets, such as ISO sizes A4, B4, B5, etc. This may be done by providing several cassettes of the desired sizes in a vertically stacked arrangement and a separate feed roller set for each cassette. The feed rollers are individually driven in accordance with the desired copy sheet size. This system, although efficient, is quite expensive due to the several sets of feed rollers and interconnecting mechanism.
It is also possible to provide several copy sheet sizes using a single set of feed rollers. In one case cassettes of different sizes are provided and selectively inserted in the copying machine frame below the feed rollers. In another system a single cassette is provided and the stack of copy sheets is placed in the cassette in alignment with a front edge and a side edge of the cassette. In each case, it is necessary to provide a separate feed roller for the other side edge of the copy sheet corresponding to each sheet size. For example, if the copying machine is designed to utilize A4, B4 and B5 size sheets, there must be at least four feed rollers; one for the aligned side edge and three for the non-aligned side edges. The disadvantage of this arrangement is that a rotational moment is imparted to the sheet for the larger sheet sizes due to the fact that more than one feed roller engages the non-aligned side edge portion of the sheet resulting in skew feed.
A prior art proposal to overcome this problem is to slidably mount a single non-aligned side edge feed roller and position it in accordance with the sheet size. If the roller is positioned automatically, a complicated mechanism is required. If the roller is positioned manually, it will often be moved to the wrong position or neglected by inexperienced or careless operators with the inevitable result of skew feed.
In most conventional copying machines the cassettes are partially inserted from the side and protrude from the side of the copying machine. This prevents the copying machine from being installed in a compact space in an office since additional space must be provided at the side of the copying machine for the cassettes and even more space must be provided to allow the cassettes to be inserted and detached from the machine. The cassettes are inserted from the side in such copying machines since the insertion and detachment direction must be parallel to the feed direction. The wasted space inherent in such a system is undesirable.